During tumor progression the well-ordered architecture of the mammary gland becomes disrupted as hyperproliferative cells accumulate in the luminal space of ducts and lobules. Over time the neoplastic cells can invade into the stroma where they can access the vasculature system and disseminate to distant sites throughout the body. Although the induction of invasive growth is a key step in the metastatic cascade, how breast cancer cells become invasive remains poorly understood. Using an organotypic culture system and animal models we propose to determine (1) how cell motility is regulated by intracellular signaling pathways during collective invasion; (2) how mammary fibroblasts induce collective invasion and (3) how subpopulations of neoplastic cells trigger collective invasion. Deciphering the requirements for breast cancer invasion could identify targets for therapeutic intervention in both the tumor and the microenvironment.